1. Field of the Invention
The present invention relates generally to microwave and millimeter-wave hybrid and monolithic integrated circuits, and more particularly to such circuits employing coplanar waveguide. Specifically, the present invention relates to circuits which combine coplanar waveguides and slotlines for constructing system components.
2. Description of the Related Art
There is an increasing demand for microwave and millimeter-wave hybrid and monolithic integrated circuits in many system applications. Due to their planar nature, these circuits offer cost, weight, reliability and reproducibility advantages when combined with photolithographic techniques. Microstrip has been the transmission line typically used in microwave circuit design partly because of the vast amount of design information available. Microstrip, however, has a number of disadvantages.
Although semiconductor devices can be readily integrated in series with the microstrip, shunt devices must be mounted by drilling through the substrate. Drilling adds cost and discontinuities which increase in the millimeter-wave region where tolerances become critical. Furthermore, the microstrip impedance and guided-wavelength characteristics are very sensitive to substrate thickness which further increases design problems at higher frequencies.
Coplanar waveguide (CPW) is an alternative transmission line that is truly planar and allows easy series and shunt device mounting CPW is not very sensitive to substrate thickness and allows a wide range of impedance values on relatively thick substrates. The radiation loss in the CPW odd mode is low for an open transmission line. These characteristics make CPW important for millimeter-wave circuits and has stirred considerable interest in microwave and millimeter-wave integrated circuit design.
Although the characteristics and relative advantages of CPW are known, only a limited number of CPW components are available for circuit design. These components include directional couplers, mixers, diplexers, and end-coupled resonant CPW filters.